It is proposed to study the structures of SV-40 and/or polyoma viruses by means of single crystal X-ray diffraction, X-ray solution scattering, and electron microscopy. SV-40 and polyoma are closely related, tumorogenic, mammalian viruses that have received intensive genetic and biochemical study. They are of particular structural interest because their genome is organized much like eukaryotic chromatin, with cellular histones being bound to viral DNA to produce a typical nucleosomal pattern. Knowledge of the structure of chromatin is crucial for the understanding of gene regulation, transcription and other cell processes. X-ray crystallography is still the most powerful tool available for the visualization of molecular structure. High resolution images of two plant viruses have recently been published, confirming that structures weighing millions of Daltons can be successfully analyzed. Crystals of polyoma virus have been grown by Murakami, and are now under study in the laboratory of Donald Caspar, with whom I am collaborating. I have begun experiments to crystallize SV-40, and promising crystals, as yet too small for X-ray diffraction, have been grown. Another technique capable of yielding useful images is the low dose electron microscopy pioneered by Unwin and Henderson. This technique uses two-dimensional crystals as specimens. Noise in the image is reduced by the effective averaging over many unit cells, and radiation damage is diminished because the electron dose that would normally pass through one element (molecule, virus etc.) is distributed over the whole array. For specimens as thick as viruses multiple scattering limits this technique to medium resolution. True two-dimensional crystals of viruses have been observed.